Oxidative damage to DNA in cell cultures of bronchial epithelial cells is used as a model to study the mechanism for such damage in vitro. Oxidant-induced damage may be significant in vivo due to challenges presented from free radicals generated from cigarette smoke. Primary human bronchial epithelial cells, an immortalized cell line derived from these cells (Beas-12) and the HUT 292 tumor cell line are exposed to the model oxidants, hydrogen peroxide and menadione, or to cigarette smoke- conditioned media then assayed for biochemical alterations and oxidative DNA damage. Biochemical markers include glutathione, glutathione peroxidase, glutathione reductase, intracellular Ca++ mobilization, peroxide levels and cell viability. Representative measures of DNA damage are 8-OH-deoxyguanosine (8-OH-dG) and thymine glycol content. These measures allow the examination of the hypothesis that substantial oxidative DNA damage does not occur until the cellular protective mechanisms for scavenging active oxygen species are compromised or depleted. The response of human bronchial epithelial cells to oxidant-induced stress caused by components in cigarette smoke may have importance in the etiology of lung cancer caused by smoking.